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A URI scheme is the top level of the URI (Uniform Resource Identifier) naming structure. All URIs and absolute URI references are formed with a scheme name, followed by a colon character, and the remainder of the URI called (in the outdated RFCs 1738 and 2396, but not the current STD 66/RFC 3986) the scheme-specific part. The syntax and semantics of the scheme-specific part are left largely to the specifications governing individual schemes, subject to certain constraints such as reserved characters and how to "escape" them.

URI schemes are sometimes erroneously referred to as "protocols", or specifically as URI protocols or URL protocols, since most were originally designed to be used with a particular protocol, and often have the same name. The http: scheme, for instance, is generally used for interacting with Web resources using HyperText Transfer Protocol. Today, URIs with that scheme are also used for other purposes, such as RDF resource identifiers and XML namespaces, that are not related to the protocol. Furthermore, some URI schemes are not associated with any specific protocol (e.g. "file:") and many others do not use the name of a protocol as their prefix (e.g. "news:").

URI schemes should be registered with IANA, although many non-registered schemes are used in practice. RFC 2717 describes the procedures for registering new URI schemes. RFC 2718 provides guidelines for the development of new URI schemes. Both of these RFCs are currently undergoing revision.

Contents 1 Generic syntax 1.1 Examples 2 Official IANA-registered schemes 3 Unofficial but common URI schemes 4 External links

Internet standard STD 66 (also RFC 3986) defines the generic syntax to be used in all URI schemes. Every URI is defined as consisting of four parts, as follows:

 :  [ ?  ] [ #  ]

The scheme name consist of a letter followed by any combination of letters, digits, and the plus ("+"), period ("."), or hyphen ("-") characters; and is terminated by a colon (":").

The hierarchical part of the URI is intended to hold identification information hierarchical in nature. Usually this part begins with a double forward slash ("//"), followed by an authority part and an optional path. The authority part holds an optional user information part terminated with "@" (e.g. username:password@), a hostname (i.e. domain name or IP address), and an optional port number preceded by a colon ":". The path part is a sequence of segments (conceptually similar to directories, though not necessarily representing them) separated by a forward slash ("/"). Each segment can contain parameters separated from it using a semicolon (";"), though this is rarely used in practice.

The query is an optional part separated with a question mark, which contains additional identification information which is not hierarchical in nature. Its syntax is not generically defined, but is commonly organized as a sequence of = pairs separated by an ampersand, e. g. key1=value1&key2=value2&key3=value3.

The fragment is an optional part separated from the front parts by a hash ("#"). It holds additional identifying information which allows indirect identification of a secondary resource, e.g. a section heading in an article identified by the remainder of the URI.

The following are two example URIs and their component parts (taken from STD 66):

  foo://example.com:8042/over/there?name=ferret#nose
  \ /   \______________/\_________/ \_________/ \__/
   |           |             |           |        |
scheme     authority        path       query   fragment
   |   ______________________|_
  / \ /                        \
  urn:example:animal:ferret:nose

The official URI schemes registered with the IANA follow.

Scheme	Purpose	Defined by	General format	Notes
aaa:	Diameter Protocol	RFC 3588	aaa: FQDN [port] [transport] [protocol]
aaas:	Diameter Protocol with Secure Transport	RFC 3588	aaas: FQDN [port] [transport] [protocol]
acap:	application configuration access protocol	RFC 2244	acap://url-server/url-enc-entry [url-filter] [url-extension]	URL scheme used within the ACAP protocol for the "subdataset" attribute, referrals and inheritance
cap:	calendar access protocol	RFC 4324	"cap://" csidpart [ "/" relcalid ]	URL scheme used to designate both calendar stores and calendars accessible using the CAP protocol
cid:	Referencing individual parts of an SMTP/MIME message	RFC 2392	cid:	e.g. referencing an attached image within a formatted e-mail. (See also mid:)
crid:	TV-Anytime Content Reference Identifier	RFC 4078	crid:///	Allow references to scheduled publications of broadcast media content.
data:	Inclusion of small data items inline	RFC 2397	data:;[base64,]
dav:	HTTP Extensions for Distributed Authoring (WebDAV)	RFC 2518	dav:	Used for internal identifiers only; WebDAV itself addresses resources using the http: and https: schemes. [1]
dict:	dictionary service protocol	RFC 2229	dict://;@:/d::: dict://;@:/m::::	refer to definitions or word lists available using the DICT protocol
dns:	Domain Name System	RFC 4501	dns://dnsauthority/dnsname?dnsquery	designates a DNS resource record set, referenced by domain name, class, type, and, optionally, the authority
fax:	used for telefacsimile numbers	RFC 2806	fax:	Seems to be deprecated in RFC 3966 in favour of tel:
file:	addressing files on local or network file systems	RFC 1738	generic syntax (often appears as file:///path, the 3rd '/' representing the root directory of the local system)	Unusual in not being bound to any network protocol, and not usable in an Internet context.
ftp:	FTP resources	RFC 1738	generic syntax
go:		RFC 4266	go://[]?[]*[;=[,]] go:*[;=[,]]
gopher:	used with Gopher protocol	RFC 4266	gopher://://
http:	HTTP resources	RFC 2616	generic syntax
https:	HTTP connections secured using SSL/TLS	RFC 2817	generic syntax
imap:	Accessing e-mail resources through IMAP	RFC 2192
ldap:	LDAP directory request	RFC 2255 RFC 4516
mailto:	SMTP e-mail addresses and default content	RFC 2368	mailto: ?header1=value1&header2=value2 example: mailto:jsmith@example.com?subject=A%20Test&body=My%20idea%20is%3A%20%0A	Headers are optional, but often include subject=; body= can be used to pre-fill the body of the message.
mid:	Referencing SMTP/MIME messages, or parts of messages.	RFC 2392	mid: or mid:/	(See also cid:)
news:	(Usenet) newsgroups and postings	RFC 1738	news: or news:	References a particular resource, regardless of location.
nfs:	Network File System resources	RFC 2224	generic syntax
nntp:	Usenet NNTP	RFC 1738	nntp://://	Referencing a specific host is often less useful than referencing the resource generically, as NNTP servers are not always publicly accessible
pop:	accessing mailbox through POP3	RFC 2384
pres:	Used in Common Profile for Presence (CPP) to identify presence	RFC 3859	pres: ?header1=value1&header2=value2	Similar to "mailto:"
sip:	used with Session Initiation Protocol (SIP)	RFC 3969 RFC 3261
sips:	Secure equivalent of sip:	RFC 3969 RFC 3261
snmp:	Simple Network Management Protocol	RFC 4088	snmp://[user@]host[:port][/[[;]][/]] examples: snmp://example.com//1.3.6.1.2.1.1.3+ snmp://tester5@example.com:8161/bridge1;800002b804616263
tel:	used for telephone numbers	RFC 3966 RFC 2806	tel:
telnet:	used with telnet	RFC 4248	telnet://:@[:/]
urn:	Uniform Resource Names	RFC 2141	urn::
wais:	used with Wide area information server (WAIS)	RFC 4156
xmpp:	XMPP (Jabber)	RFC 4622	xmpp: /?	xmpp://... is also allowed, xmpp: is the simplest form

Scheme	Purpose	Defined by	General format	Notes
about:	Displaying product information and internal information	Un-standardised		Widely used by web browsers, sometimes even providing interactive resources
aim:	Launching AOL Instant Messenger conversations	AOL	aim:goim?screenname=&message=
callto:	Launching Skype call (unofficial; see also skype:)	Skype	callto: or callto: [2]	Works with current version of Skype with Firefox, Internet Explorer and Safari
ed2k:	resources available using the eDonkey2000 network	eDonkey2000	ed2k://|file||||/	Links to servers are also possible, as are additional parameters. Official documentation from eDonkey2000 website
feed:	web feed subscription		feed:[//]
irc:	Connecting to a server, joining a channel	IETF Draft Old IETF Draft	irc://[:]/[[?]]	"host" may optionally be an IRC network name, assuming the client knows a server associated with the name
ircs:	Secure equivalent of irc:	IETF Draft	ircs://[:]/[[?]]	See irc:
lastfm:	Launching Lastfm music client	Lastfm	lastfm://globaltags/ or lastfm://user//
magnet:	"magnet links"	Magnet-URI Project	magnet:?xt=urn:sha1:&dn= (other parameters are also possible)	Used by various peer-to-peer clients, usually providing the hash of a file to be located on the network.
mms:	Windows streaming media		mms://:/	Used by Windows Media Player to stream audio and/or video.
msnim:	Adding a contact, or starting a conversation in Windows Live Messenger	Windows Live Messenger	msnim:chat?contact=
psyc:	Used to identify or locate a person, group, place or a service and specify its ability to communicate	PSYC	psyc:[//[:[][]]/[][#]	Official documentation from PSYC website
rsync:	Rsync		rsync://[:]/
skype:	Launching Skype call (official; see also callto:)	Skype	skype:?	Official documentation from Skype website
ssh:	SSH connections (like telnet:) and	IETF Draft	ssh://[[;fingerprint=]@][:]
sftp:	SFTP file transfers (not be to confused with FTPS (FTP/SSL))	IETF Draft	sftp://[[;fingerprint=]@][:]//
smb:	Accessing Samba shares (like NFS), see CIFS, Windows file sharing etc.	IETF Draft	smb://[@]host[:][/[]][?=;=] or smb://[@]workgroup[:][/]
soldat:	Joining servers	Soldat	soldat://:/ example: soldat://127.0.0.1:23073/	Official note in Manual
steam:	Interact with Steam: install apps, purchase games, run games, etc.	Steam, Valve Corporation	steam: or steam:///	Official documentation from Valve Developer Community website
unreal:	Joining servers	Unreal	unreal://[:]/	Unreal legacy "protocol"
ut2004:	Joining servers	Unreal Tournament 2004	ut2004://[:][/map?options]	Documentation from Unreal Developer Network
xfire:	Adding friends and servers, joining servers, changing status text.	Xfire	xfire:?=&=	Official documentation from Xfire website
ymsgr:	Sending an instant message to a Yahoo! Contact.	Yahoo! Instant Messenger With Voice	ymsgr:sendIM?

• Official IANA Registry of URI Schemes
• More information, including many more schemes